The present invention generally relates to aerospace systems, and more particularly, to a cabin pressure control system with multiple outflow valves and a method of calibrating the outflow valve position feedback during flight.
Some aircraft have multiple outflow valves (OFVs) that are controlled together to effect an airflow proportional bias between them. Sometimes large aircraft have two OFVs, but some have more. For accurate flow biasing, which may be required over the full range of airflow available from the engines and over the varying commanded proportions from the ventilation control system, very accurate calibration of the OFV position sensors is required.
This can result in costly position feedback components, expensive high precision circuitry to read the output, and difficult/costly calibration procedures; either in the shop or on the airplane.
However, often the valve position sensor cannot be calibrated accurately enough to maintain proper OFV biasing. Further, circuit errors may contribute to OFV position error. Additional errors due to temperature effects or circuitry component drift over time may cause improper operation of OFVs.
Additional errors may be introduced for thrust recovery valves (TRV). For example, if the OFVs are TRV type, then they are mounted on the skin of the airplane. TRVs have multiple loads on the valve due to aerodynamic effects. Therefore, the doors of the valve may deform during flight creating position error of the TRV. Further, TRVs often have linkages between the rotary actuator and the valve doors. These linkages may have variation in them (including effects of wear over time), and there are secondary effects that cause additional errors in the position sensor, (which may be mounted on the rotary actuator shaft) and the actual door positions.
As can be seen, there is a need for an improved method of calibrating outflow valve position on an aircraft.